Long-Wavelength Imaging Radar - A Window on Near-Surface Processes

Radar observations of the terrestrial planets and the Galilean satellites have often been used to study the roughness and dielectric properties of their surfaces and near-surface environments. Longer radar wavelengths are most effective for deep probing, and can reveal subtle variations in bulk chemistry, the population of suspended rocks or voids, and sub-surface geologic features. We report here on applications of new L- and P-band (24-70 cm wavelength) radar observations of the Moon and Mars-analog terrestrial sites. The new 70-cm lunar observations have a spatial resolution of 300 m, representing a 10-fold improvement over previous maps. Images of the lunar poles do not support the existence of thick, Mercury-like deposits of ice in permanently shadowed craters. Any ice within the radar-observable areas must thus occur as disseminated grains or thin interbedded layers within the regolith. The new images also provide much greater spatial detail of geochemical differences among the mare basalt flows, and regional variations across the southern highlands that appear to correlate with large basin ejecta deposits. L- and P-band AIRSAR images and L-band SIR-C images, for a number of arid or semi-arid regions (Hawaii, Death Valley, Northern Arizona, Egypt), are being used to test theoretical predictions of backscatter from volume scatterers and buried surfaces. We are supporting these imaging radar studies with field topography measurements, ground-penetrating radar surveys, and laboratory sample analysis to constrain the sources and expected polarimetric properties of surface and sub-surface echoes. This work is an important step in refining the requirements on orbital radar systems for Mars and the Galilean satellites. Part of the research described in this paper was carried out by the Jet Propulsion Laboratory, California Institute of Technology, under a contract with NASA.